The ISIS killer known as "Jihadi John" was targeted by a drone strike. (Uncredited/AP)

The Pentagon said in a statement that the military was "assessing the results" of the strike as Secretary of State John Kerry and Prime Minister David Cameron hedged on sickexecutioner Mohammed Emwazi's demise, both saying in separate briefings that his fate was not yet clear.

"We are still assessing the results of this strike, but the terrorists associated with Daesh need to know this: Your days are numbered, and you will be defeated," said Kerry from Tunis, using an Arabic term for ISIS. "There is no future, no path forward for Daesh, which does not lead ultimately to its elimination, to its destruction."

From London, meanwhile, Cameron, who described ISIS as an "evil terrorist death cult" and Emwazi himself as a "barbaric murderer," said the British goverment had been working with American officials around the clock while defending the decision to target the crazed head hunter.

"(Emwazi) posed an ongoing and serious threat. This (airstrike) was an act of self-defense. It was the right thing to do," he said.

“John,” whose real name is Mohammed Emwazi, was born in Kuwait, but raised in London.

Emwazi was believed to have been killed as he left a building to enter a vehicle.

The savage extremist has been a top target for Western officials since he appeared in a series of sickening, professional-looking propaganda videos for the Islamic terrorist group, including a 2014 clip showing the beheading of American journalist James Foley.

If Emwazi was killed in the air strike near Raqqa, Syria, Thursday, he was most likely vaporized, according to officials.

A black-clad Emwazi soon became the familiar British-accented voice of terror.

The masked madman has not been seen by his family since 2013, when the jihadist told his parents he was moving to Syria to "deliver aid."

In a video released in August, the ISIS spokesman pledged to return to London — to kill more British citizens.

Advertisement

"I will soon go back to Britain," he said. "I will carry on cutting heads."

"Jihadi John" is seen in a 2014 video with American journalist James Foley. (Youtube)

Emwazi is also believed to have participated in the executions of American journalist Steven Sotloff; aid worker Abdul-Rahman Kassig; British aid workers David Haines and Alan Henning, and Japanese journalist Kenji Goto.

"If they got him great," she said. But "it doesn't bring my son back."

“Jihadi John” stands next to Steven Sotloff in this still image from a video obtained from SITE Intel Group website on February 26, 2015. (REUTERS TV/REUTERS)

Emwazi attended school in London after moving with his family from Kuwait and studied computer science at the University of Westminster.

He first came to the attention of the British authorities in 2009, after he graduated from college.

Court documents from 2011 confirm Emwazi was on authorities' radar, linked to a group involved in gathering money and equipment "for terrorism-related purposes" in Somalia.

He was among at least 12 people suspected of belonging to a network sympathetic to Somalia-based Al Shabab, the militant Islamist organization that pledged allegiance to Al Qaeda a year later.

Shirley Sotloff, the mother of murdered journalist Steven Sotloff, said it's great if Emwazi was killed, but it wouldn't bring her son back. (CNN)

That network also had links to would-be terrorists in London.

Emwazi was described by a former hostage as a bloodthirsty psychopath who especially enjoyed threatening Western hostages.

Spanish journalist Javier Espinosa, who was held in Syria for more than six months after being abducted in September 2013, said Emwazi would explain in gory detail how the militants would carry out a beheading.

Captives being held by three British-sounding captors nicknamed them "the Beatles," Espinosa said.